The present invention relates to spacecraft attitude control. In particular, the present invention relates to a method and system for detecting and correcting spacecraft yaw errors due to sun sensor alignment uncertainty.
Accurately maintaining spacecraft attitude pointing is critical to payload communication performance. To achieve this goal, the on-board control system first has to accurately determine the spacecraft""s actual attitude in space using data from on-board sensors such as the earth sensor, the sun sensor, or the star tracker. Since sensor data is used as the reference for spacecraft attitude determination, the accuracy of the sensor alignments has a significant effect on the spacecraft""s attitude determination performance.
Errors always exist in sensor alignments. Common error sources include measurement errors introduced during the manufacturing phase, sensor alignment shifts that occur during the launch, and thermal distortions while in orbit. Because each sensor will have a different alignment error, different sensors can produce conflicting information. As a result, attitude determination performance will be degraded and an attitude disturbance will be produced.
It is desirable to have a system that can estimate such sensor alignment uncertainties and compensate for the errors introduced. Since the alignment uncertainties can be time varying, an on-board estimation-compensation system is necessary. With such a system, spacecraft pointing performance is greatly improved.
The present invention provides a method for correcting differences in measurements between sensors. According to the method, misalignment of a roll error of a sun sensor alignment is estimated with respect to a reference roll value measured by an earth sensor. Sun sensor assembly elevation angle residual is calculated utilizing the estimated sun sensor assembly roll misalignment. A yaw attitude of the spacecraft is updated based upon the calculated sun sensor assembly elevation residual.
The present invention also includes a spacecraft attitude control system that includes a controller operable to carry out the above-described method and at least one element operable to alter the yaw attitude of the spacecraft.